Tech Stuff

Let’s face reality. Most drivers don’t know their car battery is going bad until their car doesn’t start. That’s when being a professional can separate you from the DIYer who simply replaces the battery without any diagnostic testing. Remember to ask the driver of the vehicle probing questions to see if the problem is really just a worn-out battery, or if something else is happening. At this point, it’s time to do your diagnostic check, and also explain why this is important to prevent a comeback. Make sure the customer knows you provided a service, and didn’t just replace the battery.

Sometimes diagnosing a no-crank/no-start problem can be more difficult than diagnosing a driveability problem. The average passenger car or light truck has at least two dozen electronic control units on-board, and luxury vehicles can have three times that many. Some of those tiny electronic brains can make a decision that will drain the battery almost overnight.

For driveability diagnostics, we at least have the advantage of OBD II trouble codes that are the same on every vehicle. Electrical systems and electronic equipment can be different from one vehicle to the next, even on the same model in the same model year. This makes it harder to answer the two questions you face with every no-start: Why is this battery dead, and should it be replaced?

Take your readings directly from the battery posts, not the clamps. This ensures a more accurate reading.

One of the most common tools in your shop can help you find the answers to battery questions: a digital volt-ohm meter (DVOM). You’ll need more than this to test the battery itself, but you can test everything that draws current from the battery with a DVOM, and that can tell you if the battery’s demise was natural or premature.

Opening shot

Usually the first sign of battery trouble is slow cranking speed during starting. That doesn’t automatically mean the battery is discharged though, because the same symptom could be caused by high-resistance in the starter circuit. Unfortunately, most people don’t notice slow cranking speed because fuel injection and electronic controls will usually get the engine started after just a few compression strokes. As a result, their first obvious clue to a battery problem is when the engine won’t crank over at all, and that’s when the car ends up in your bay.

So, is it a bad battery or a bad connection somewhere in the starting system? With a DVOM that records min/max values, you can run a quick and reliable test that will point you in the right direction. But first you need to charge the battery, and you should do it without disconnecting the battery cables so you don’t erase any data that is hopefully still stored in the powertrain control module (PCM). When that’s done, disconnect the charger and connect the meter to the battery terminals; not to the cable clamps and not to chassis ground, but directly to the battery posts. Now you’re ready for the test.

Read the open-circuit battery voltage. A fully charged battery will read 12.6 volts, and a battery with only 50% charge will read 12.3 volts. After charging a battery, the initial reading may be higher than the battery’s true state-of-charge due to an affect called “surface charge.”

With the voltmeter set to capture min/max readings, watch the meter with the engine running. About one minute after start-up, our charging system voltage went from a peak of almost 14.2 down to the normal 13.8 volts. If the charging voltage stays high, test the battery.

Turning on the headlights for just a minute or two will give you a more accurate reading of the battery’s true charge. Now turn the lights off, start the engine and note the charging voltage. After about one minute at idle, note the voltage again, then turn the engine off and read the minimum and maximum voltages recorded by the meter.

The maximum number is charging system voltage. It will probably start high, and depending on the vehicle and the condition of the battery, it should stabilize quickly at about 13.8 volts. If it’s below 13.5, look for a bad battery connection or a charging system problem. If it’s higher than 14.2 and stays high with the engine at idle, suspect a battery problem.

The minimum voltage occurs as the starter draws current from the battery. This reading is right in the middle of the normal range. A low reading indicates battery problems and a high reading indicates high resistance in the starter circuit.

The minimum number will be battery voltage during peak starter draw. If it dropped below about 9.5 volts during cranking, it’s time to test the battery itself. If the minimum is below 9 volts, that could explain a loss of data in the PCM, like learned idle speed or long-term fuel, and you might also see strange problems with some of the other control units.

If the minimum number is above about 10.8 volts, suspect high resistance somewhere in the starter circuit. Remember, these readings are taken from the battery posts, so start by checking voltage drop across the posts and the clamps. Even if everything looks clean, there could still be corrosion on the inside of the clamp that you can’t see.

This is not a definitive test. It’s just meant to point you in a diagnostic direction. But remember, a no-crank symptom doesn’t automatically mean there’s something wrong with the battery or starting system; it could be caused by mechanical problems.

Next time you see a connection coated with green fuzz instead of this nice red grease, check the voltage drop across the battery post and clamp. This one is a perfect zero millivolts.

Parasitic draw

The most common cause of premature battery failure is what we call “parasitic draw,” referring to something that continues to draw current from the battery after the ignition switch is turned off. Some current draw after shut-down is normal. The PCM and other control units are still turned on or “awake” after shut-down, and total current draw can be more than 500 milliamps for half an hour or longer. Even after all the control units turn off or “go to sleep,” the PCM continues drawing current to maintain volatile memory of fault codes, freeze frame, learned idle speed, etc. Only a few manufacturers list the key-off current draw in their service information, so for those that don’t, most techs consider 50 milliamps to be the maximum allowable parasitic draw 45 minutes after shut-down.

You can test for parasitic draw with a DVOM that reads current, but be careful. First you have to get the meter connected in series between the battery and negative battery cable without breaking the connection to the car and erasing the volatile memory in the PCM. You also have to make sure the ignition switch is not turned on with the meter connected. That’s because most meters are rated for 10 amps, and since the fuel pump runs when the ignition is first turned on, it could blow the meter’s fuse.

If you have a low-amp probe that connects to your meter, none of this is a problem; just clamp the probe around the negative battery cable. With the Key On Engine Off (KOEO), you’ll see the current draw of all the control units. With everything turned off and all the doors closed, you can watch current draw decrease in steps as each control unit times out and goes to sleep.

If the vehicle has keyless entry, the control unit may stay awake longer with the key fob in the car. Move the key fob at least 20 feet away. If you’re certain everything is off but current draw stays above the manufacturer’s specification (or above 50 milliamps) for more than 45 minutes, it’s time to check each circuit individually.

Five minutes after shut-down, a “mystery” draw was still pulling 164 milliamps of current on our customer’s Acura (the low-amp probe is set to 1 mV/10mA). And then after about 15 minutes, current draw suddenly dropped to 30 mA.

There is one more thing to watch for when measuring parasitic draw. On many vehicles the PCM will wake up sometime after shut-down to pressure test the evaporative emissions control system (EVAP). It will draw enough current to operate a leak detection pump (LDP) and/or a few solenoid valves, but it shouldn’t be enough to blow the fuse in a DVOM.

Circuit testing

You can check for current draw of individual circuits at the fuse box. Start with aftermarket equipment like a location tracking device, a flight recorder plugged into the data link connector (DLC), or any aftermarket or OEM equipment with a Bluetooth connection, like hands-free cell phone links.

Next, look for aftermarket or OEM items located somewhere away from the driver’s cockpit. For instance, we learned about a minivan with a back-seat DVD player that stayed on all night because someone tried to insert a second disc into the slot. The player kept cycling the load/eject motor because no one removed the extra disc after it ejected, and next morning the engine would not crank.

Measure the voltage drop across each fuse to find the circuit that’s drawing current. One tenth of a millivolt (0.0001 volts) across a 30-amp fuse indicates 50 milliamps of current. This reading shows the control unit for the interior lights is drawing just over 1 amp through a 10-amp fuse.

Instead of removing each fuse to find the circuit that’s drawing current, it’s easier to check the voltage drop across each fuse with a DVOM. Any time current flows through a circuit, there will be a difference in voltage between any two points in that circuit, even if the two test points are very close together. The voltage drop across a fuse might be only a few tenths of a millivolt, but if it’s more than zero, that means current is flowing through the fuse.

We found a chart published by the folks at Power Probe that lists voltage drop across standard fuses for the amount of current flowing through the fuse. For instance, in the photo on page 20 we see 8 millivolts flowing through a 10-amp fuse. According to the chart, this means 1.039 amps of current is flowing through that fuse. While that information can be useful, it’s not really necessary to know the actual number when looking for parasitic current draw. All we’re looking for is any fuse with a voltage drop greater than zero with the ignition switch turned off.

One problem with checking circuits this way is that you can’t always do it with all the doors closed. For instance, the fuse panel in that same photo is inside the car, and opening any door turns on the interior lights and wakes up some of the control units. However, we noticed after a few minutes that only the interior lights and their control unit were still drawing current.

Battery test

Once the current drain is found and fixed, you need to determine if the battery can still be trusted. But before you test it, there are a few other things to consider.

If properly cared for, a battery can live twice as long as its warranty rating. The useful life of a battery is affected by heat, vibration and depth of discharge. Consider the vehicle and how it’s typically used, and consider the age of the battery and what kind of abuse it may have suffered. Does this vehicle live in a hot climate? Is it used off-road? Is it a kid hauler used mostly for short trips by a busy parent who can’t always make sure all the lights are switched off? Sometimes you can make the decision without testing the battery.

Two different tools can be used to test a battery; a load tester and an electronic conductance tester. The load tester directly measures the battery’s ability to deliver current, while the conductance tester measures the ability of each plate to conduct electricity. Either tool can tell you if the battery is OK or if it should be replaced, but only when it’s used according to the tool maker’s instructions. If you don’t have those instructions, go to the company’s website and get them. Using these tools incorrectly will produce inaccurate test results and you’ll either condemn a good battery or, even worse, send a customer away with a bad battery.

The most common reason for inaccurate battery test results is incorrect identification of the battery. The tester compares the battery’s measured performance with its expected performance, which is expressed as cold cranking amps (CCA), reserve capacity (RC) or amp hour (Ah) rating. The tester may also consider the battery’s group size, the physical dimensions of the battery, which has an influence on the battery’s performance over time. If you can’t accurately provide the information the battery tester needs, you won’t get an accurate test.

With the circuit operating a standard 1156 light bulb, we measured a voltage drop across this fuse of 10 millivolts probing these two points. The chart indicates a current flow just shy of 3 amps.

Battery replacement

Once you’ve decided to replace the battery, be aware that you may need to use a scan tool for a charging-system-relearn procedure. Also, it’s absolutely critical to install the correct battery for that vehicle, especially if the vehicle has a stop/start system. The PCM controls the charging system, and it’s programmed for specific features like battery technology (flooded or absorbed glass mat), group size and sometimes even the electrical equipment specific to that VIN. Installing the wrong battery may prevent the charging system from working correctly and/or generate charging system fault codes. Driveability issues or problems with the gauges on the instrument panel are other signs of a mismatch between the battery and the charging system.

Battery technology is advancing rapidly, but the diagnostic techniques described here will always be useful for testing a battery that powers the starter. Since it only takes two minutes, practice the DVOM min/max test on every car that comes through your bay so you can learn what known-good looks like.

One of most useful tools you have is your own experience. ■

Just what is surface charge?

Surface charge is when the chemical reaction that produces or stores electrical energy in the battery occurs only on the surface of the lead plates. This reaction runs to completion quickly and then begins occurring deeper and more slowly in the material of the plate.

How a battery works

To make electricity, you need to make ions. An ion is any atom that has one less electron or one more electron than it normally would. This makes the atom somewhat unstable, and given the opportunity it will either accept or give up an electron to become stable again.

Whether the ions are created by manipulating magnetic fields with a generator or by a chemical reaction, electrons moving from one ionized atom to another produces the energy we call electricity.

A lead/acid battery makes electricity with a chemical reaction, but first we need to create some ions on the plates by charging the battery. When the battery is flat (discharged), the lead plates are actually lead sulfate, a compound of lead and sulfur. Charging the battery forces electrons to flow from one set of plates to the other, transforming one set into lead (with fewer electrons) and the other into lead oxide (charged with extra electrons). It also ionizes the electrolyte (sulfuric acid) so it can conduct electricity. The plates are electrically insulated from each other, so when they are connected together by connecting a circuit to the battery terminals, extra electrons flow from the positively charged plates (anode) through the circuit to the negatively charged plates (cathode). Eventually the atoms on the plates and in the acid are stable again and the chemical reaction ends.

Special thanks to Johnson Controls Power Solutions for their help with this article.

Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. He began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.